Random-coil transition

As a prelude to our binding studies, the secondary structure of aPNA itself was examined using CD spectroscopy [52]. The first aPNA to be studied was the tail-to-tail bl dimer, [Ac-Cys-Gly-Ser -Asp-Ala-Glu-Ser -Ala-Ala-Lys-Ser -Ala-Ala-Glu-Ser -Ala-Aib-Ala-Ser -Lys-Gly-NH2]2- The far-UV CD spectra of this aPNA in water at 30 °C showed the double minimum at 220 nm (n-n transition) and 206 nm (n-n transition) as well as the maximum at 193 nm (n-n transition), characteristic of a peptide a-hehx. Upon increasing the temperature, the intensity of the minimum at 200 nm decreased indicating a transition from a-helix to random stracture. An isodichroic point at 202 nm was suggestive of a temperature-depen-dent a-helix to random coil transition. The helical content of this T5(bl)-dimer at 20°C in water was estimated to be 26% [40]. 

Pii-8, like Pii-5, is designed to self-assemble at high pH. Comparable shifts in properties were found when studying Pn-8 solutions in 130 mM NaCl (Fig. 12). The p-sheet to random coil transition is lowered by 3 pD units and the pD range over which it forms nematic gels is extended down to pD 8.5 in aqueous NaCl... 

Indeed, 13C spin-lattice relaxation times can also reflect conformational changes of a protein, i.e. helix to random coil transitions. This was demonstrated with models of polyamino acids [178-180], in which definite conformations can be generated, e.g. by addition of chemicals or by changes in temperature. Thus effective molecular correlation times tc determined from spin-lattice relaxation times and the NOE factors were 24-32 ns/rad for the a carbons of poly-(/f-benzyl L-glutamate) in the more rigid helical form and about 0.8 ms/rad for the more flexible random coil form [180],... 

B) Schematic presentation of the a-helix/random-coil transition of the photoisomerizable polymer 12. 

It remains for us to discuss the dimensions of polypeptides and other helix-forming chain molecules. Most of the theoretical works on models of such chains have been primarily concerned with the equilibrium of the helix-random coil transition and have not specifically treated the chain dimensions. An exception is found in the work of Nagai (195 ), who combined his theory of the transition with a very simple model of the chain dimensions. It amounts to the assumption that each helical sequence behaves like a rigid statistical chain element without correlation in direction with the randomly coiled sections which are adjacent to it. Then, if a fraction f of the monomer units are members of helical sequences, we may at once write... 

L. Peller, On a model for the helix-random coil transition in polypeptide. I. J. Phys. Chem. 63 1194-1199 (1959). 

Since the macromolecular form must be less orderly and different from the a form, the random-coil form seems plausible. We calculated the a-helix, random coil transition enthalpy from the difference of the spreading enthalpies of the forms on supports I and II (Table V) the average is 830 cal/mole of monomeric unit. Table V also shows the transition entropies calculated as the difference between the spreading entropies of the two forms the average is 3.0 eu. The AH and the single values agree well with those found by others using different methods (42, 43, 44,45, 46, 47, 48, 49,50,51,52, 53) in the bulk phase. 

The Helix-to-Random-Coil Transition Nucleic Acid Denaturation... 

The kinetic probes 1 and 2 were selected because the hydrolysis of substrates is characterized by pH-independent rates between pH = 2-6. This is also the pH region in which at-PMAA undergoes a compact-coil to random-coil transition. By changing the hydrophobicity of substrates through variation of R, / i and R2 in I and 2, it is also possible to elucidate the role of hydrophobic microdomains inside PMAA. 

The research on polypeptides and their assembly behaviors is important and beneficial for several areas. First, polypeptides can be used as a model polymer with various chain rigidities. Polypeptides can adopt conformations of a-helix, p-sheet and random coU, which can transform into each other under controlled conditions. The a-helix to random coil transition in solutions is especially interesting. Thus, polypeptides can serve as an ideal model for investigating the influence of polymer rigidity on the assembly behavior of polymers. Second, the synthetic polypeptides... 

Physical States of Biomolecules Calorimetric Study of Helix-Random Coil Transitions in Solution... 

Later work by Carlstrom, Miller and Bryant has thrown some doubt on to whether the reaction product obtained by Bryant actually was compound (I) above [424], Attempts to repeat the synthesis of (I) met with uniform failure and frequently yielded instead a silver-containing polyelectrolyte of unknown structure (silver ion was used as a catalyst). It seems quite possible that the line width changes observed by Bryant [422] reflect an entirely different process than a helix-random coil transition. 

Transition Temperatures for Helix-Random Coil Transition (n THE Absence op Side-Chain Hydrogen Bonds"... 

Although AFobs, AHob., and ASob are extensive quantities, the value of Tit will be unaffected if several, say 91, helices cooperate in an all-or-none helix-random coil transition. However, the sharpness will be increased since da/dT)Ti, of Eq. (IV-55) will then be proportional to 91 (Loeb and Scher-aga, 1961). 

Helix-random coil transitions can also be induced in polypeptides con-... 

Recent work in the short wavelength region indicates that there may be a potential application of ultraviolet difference spectrophotometry at short wavelengths to studies of helix-random coil transitions (Imahori and Tanaka, 1959 Gla-zer and Smith, 1960). 

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